A study on homogenites and other event beds from perched basins offshore SW Taiwan: reconstructing recurrence intervals and lithofacies analysis

Three giant piston cores, MD18-3548 (20.08 m), MD18-3552 (45.98 m), and MD18-3547 (35.27 m), were collected from perched basins in offshore SW Taiwan, with the objective of studying event beds. Detailed grain-size analysis (1 cm resolution), 14C AMS dating, and X-CT-scan of the above-mentioned cores were performed, and the lithofacies were carefully delineated. Four distinct lithofacies were identified, namely hemipelagic sediments, homogenites with a silty basal layer, turbidites, and thin silty layers. Homogenites were documented for the first time from offshore SW Taiwan, and they considerably differed from the homogenites reported in other regions of the world, leading to a new facies description: “homogenites with a silty basal layer”. All homogenite units are floored by a thin (usually less than 10 cm thick), coarsening-upward first and then fining-upward unit, capped by a thick structureless mud devoid of bioturbation. A total of 49 14C AMS dating were carried out from planktonic foraminfera. The depositional ages of the event beds were interpolated from the sedimentation rate of hemipelagites with the youngest event being ~70 BP cal yrs and the oldest event being ~23 BP cal kyrs. The average thickness of homogenite units is ~100 cm, while the thickest homogenite is ~225 cm, and the thinnest homogenite is ~40 cm.We consider large earthquakes to be responsible for the deposition of homogenite units, as the recent 2006 Hengchun doublet quake (7.0 Mw) did not produce any homogenites in our location. In offshore SW Taiwan, the splay fault or out-of-sequence-thrust fault are causative faults for large destructive earthquakes. In total, seventy-one event beds were identified from the above-mentioned cores, twenty-one homogenites with a silty basal layer, twenty-four turbidites, and twenty-six thin silty layers. Three major clusters, along with two outliers, were identified. The average recurrence (excluding the outliers) interval for homogenites with a silty basal layer is ~200-2000 years, whereas the average recurrence interval for turbidites and thin silty layers is ~200-500 years. The average recurrence interval of all the event beds interval for all the three cores is ~534 years.Keywords: perched basins; homogenites; event bed clusters; X-CT image; 14C dating; grain size